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`Fifteen' was getting the parity wrong on any size of board where
the top left and bottom right corners didn't have the same
chessboard colour.

[originally from svn r4185]

--- a/fifteen.c

+++ b/fifteen.c

@@ -199,15 +199,16 @@

      * Determine the required parity of the overall permutation.

      * This is the XOR of:

      * 

-     *  - The chessboard parity ((x^y)&1) of the gap square. The

-     *    bottom right, and therefore also the top left, count as

-     *    even.

+     * 	- The chessboard parity ((x^y)&1) of the gap square. The

+     * 	  bottom right counts as even.

      * 

      *  - The parity of n. (The target permutation is 1,...,n-1,0

      *    rather than 0,...,n-1; this is a cyclic permutation of

      *    the starting point and hence is odd iff n is even.)

      */

-    parity = (X(params, gap) ^ Y(params, gap) ^ (n+1)) & 1;

+    parity = ((X(params, gap) - (params->w-1)) ^

+	      (Y(params, gap) - (params->h-1)) ^

+	      (n+1)) & 1;

     /*

      * Try the last two tiles one way round. If that fails, swap